Looking for something you're sure you won't find? Even if the item you seek is close by, a doubtful attitude reduces your chances of finding it, claims a new study funded by the US Department of Homeland Security.

Researchers simulated the conditions of an airport security checkpoint and instructed subjects to search for weapons. LiveScience reports that the monotony of the searches and increasing rarity of the weapons decreased subjects' attention spans:

They asked subjects in the lab to look at X-ray scans of checked baggage, as if they were airport screeners. In one trial, 50 percent of the scans showed a gun or knife in the bag. In this test, subjects did fairly well, only missing the weapons about 7 percent of the time.

In another trial, the guns and knives appeared much more infrequently - in only 2 percent of bags. This time, subjects missed the weapon 30 percent of the time. In other words, their accuracy went down sharply when the item they were searching for was less prevalent.

"The probability of finding something affects how quickly you abandon the search," Wolfe told LiveScience. "If you're looking for the can opener in that kitchen drawer that's full of gadgets, and you know it's probably there, you're going to spend a lot of time searching for that can opener."

But if someone suggests you search for something rare in the kitchen drawer, such as valuable Civil War-era silver, he said, you're likely to give up the hunt quickly, because you lack confidence that you'll find it.

According to researchers, a tendency to ignore unlikely discoveries is the result of evolutionary adaptation. So while this trait was evolutionarily advantageous for our mammoth-hunting ancestors, it's a bum deal for the TSA.

Luckily, now we know that the power of positive thinking can win the war on terror.